Any reason why this choice was made. If not, it does not seem to me like a good choice to move away from the Java way of declaring it. Would have made the transition from Java to Scala easier (not by much, but little bit)

x : T is the standard notation for types in logic and many programming languages. C and its descendants, with Java among them, deviates from this. But the type notation of C is really awful (try to write down the type for some moderately complicated higher order function like map).

Also, with this notation it is easy to leave out the type (as Wysawyg has already written), or to add a type inside an expression.